Magnetic recording medium and process for producing the same

Details Magnetic recording medium and process for producing the same Magnetic recording medium and process for producing the same

2003-03-27

2004-08-17

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Continuous and nonuniform or irregular surface on layer or...

C428S323000, C428S329000, C428S336000, C428S690000, C428S690000, C428S690000, C428S690000, C427S131000

Reexamination Certificate

active

06777061

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium, and more specifically to a high recording density magnetic recording medium with improved dropout and excellent surface smoothness and electromagnetic conversion characteristics.
2. Disclosure of the Related Art
In recent years, the magnetic recording media for digital recording to release from signal deterioration arising from repeated copying have become common. Furthermore, in order to cope with increases in recording data volumes, magnetic recording media with increased recording densities are required. In order to increase the recording density, factors such as media thickness loss and self-demagnetizing loss must be considered, and for these reasons, it is desired to reduce in the film thickness of the magnetic layer.
However, when the thickness of the magnetic layer is reduced, the surface roughness of the support is reflected on the surface of the magnetic layer, causing a loss of smoothness in the magnetic layer surface, and a deterioration of the electromagnetic conversion characteristics. Consequently, on the surface of the support a non-magnetic layer is provided, for example, as an undercoat layer, and the magnetic layer is then provided with this non-magnetic layer intervening. As a result, the surface of this lower non-magnetic layer provided beneath the magnetic layer must be formed with smooth.
Furthermore, during heat curing which is conducted in a rolled state during production of the magnetic medium, and during storage in a high temperature, high humidity environment following production, transfer occurs from the back coat layer to the surface of the magnetic layer, causing a deterioration in the surface smoothness of the magnetic layer, and a degrading of the electromagnetic conversion characteristics.
Japanese Patent Laid-Open Publication No. Hei 10-334450 discloses a magnetic recording medium in which the lower non-magnetic layer contains acicular particles of hydrated iron oxide dispersed in a binder, wherein the acicular particles of hydrated iron oxide have the average long axial length of 0.2 &mgr;m or less, and the average short axial length of 0.04 &mgr;m or less. However, according to the technique in this publication, by employing a lower non-magnetic layer comprising particles of hydrated iron oxide, the surface of the magnetic layer formed thereon does not form an excessively specular state, and a suitable surface state can be achieved. Specifically, the examples presented in this publication disclose magnetic recording media in which the surface roughness (Ra) of the lower non-magnetic layer is within a range from 2.3 to 2.6 nm, and the surface roughness (Ra) of the magnetic layer is from 2.5 to 2.8 nm. In other words, no investigations were conducted on ultra smooth magnetic layers with a surface roughness of 2.4 &mgr;m or less. Furthermore, the variation of the surface roughness of the magnetic layer between before and after heat curing was not evaluated. In addition, a magnetic powder with an average long axial length of 0.24 &mgr;m is used. In a magnetic powder with a long axial length of this length, the filling factor is unsatisfactory, and an ultra smooth magnetic layer is difficult to achieve. Accordingly, a medium with electric conversion characteristics capable of coping with high recording densities and with improved dropout cannot be obtained.
Japanese Patent Laid-Open Publication No. Hei 10-308014 discloses a magnetic recording medium comprising a magnetic layer of thickness 0.2 &mgr;m formed on top of a non-magnetic layer comprising iron oxyhydroxide and a binder, which is provided on a non-magnetic support. However, according to the examples in this publication, the surface roughness (Ra) of the magnetic layer is within a range from 3.1 to 6.1 &mgr;m, and no investigations were conducted on ultra smooth magnetic layers. In addition, the layered application of the non-magnetic layer and the magnetic layer is conducted using a wet-on-wet system.
Japanese Patent Laid-Open Publication No. Hei 11-3517 discloses a magnetic recording medium comprising a magnetic layer of thickness 0.2 &mgr;m formed on top of a non-magnetic layer comprising iron oxyhydroxide and a binder containing polar functional groups, which is provided on a non-magnetic support. However, according to the examples in this publication, the surface roughness (Ra) of the magnetic layer is within a range from 3.6 to 3.8 &mgr;m, and no investigations were conducted on ultra smooth magnetic layers. In addition, the layered application of the non-magnetic layer and the magnetic layer is conducted using a wet-on-wet system.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a high recording density magnetic recording medium with improved dropout and excellent surface smoothness and electromagnetic conversion characteristics, as well as a process for producing such a medium.
As a result of intensive investigation, the inventors of the present invention discovered that by using a lower non-magnetic layer containing an acicular powder of hydrated iron oxide as a non-magnetic powder, and an upper magnetic layer containing a ferromagnetic powder with an average long axial length of 0.1 &mgr;m or less, and by applying the upper magnetic layer following curing of the lower non-magnetic layer, a high recording density magnetic recording medium with improved dropout and excellent electromagnetic conversion characteristics could be obtained, with an ultra smooth magnetic layer surface with a surface roughness (Ra) of 2.4 nm or less, a level of which has proved extremely difficult to achieve with conventional techniques, and the inventors were hence able to complete the present invention.
In other words, the present invention incorporates the following aspects.
(1) A magnetic recording medium comprising a lower non-magnetic layer containing at least a non-magnetic powder and a binder resin on one surface of a non-magnetic support, an upper magnetic layer containing at least a ferromagnetic powder and a binder resin on the lower non-magnetic layer, and a back coat layer on an opposite surface of the non-magnetic support, wherein the lower non-magnetic layer contains an acicular powder of hydrated iron oxide as the non-magnetic powder, the ferromagnetic powder has an average long axial length of 0.1 &mgr;m or less, and the upper magnetic layer has a surface roughness (center line average roughness: Ra) of 2.4 nm or less.
(2) The magnetic recording medium according to the aspect (1), wherein the acicular powder of hydrated iron oxide has an average long axial length of 0.15 &mgr;m or less.
(3) The magnetic recording medium according to the aspect (1) or (2), wherein the lower non-magnetic layer has a surface roughness (center line average roughness: Ra) of 2.7 nm or less.
(4) The magnetic recording medium according to any one of the aspects (1) through (3), wherein the binder resin of the lower non-magnetic layer is a radiation curing type binder resin.
(5) The magnetic recording medium according to any one of the aspects (1) through (4), wherein the upper magnetic layer has a thickness of 0.2 &mgr;m or less.
(6) The magnetic recording medium according to any one of the aspects (1) through (5), wherein the upper magnetic layer is provided on the lower non-magnetic layer following curing of the lower non-magnetic layer.
(7) A process for producing a magnetic recording medium comprising the steps of:
forming a lower non-magnetic layer by applying a non-magnetic layer coating material containing at least a non-magnetic powder containing an acicular powder of hydrated iron oxide and a binder resin to one surf ace of a non-magnetic support, and then drying and curing the coating material;
forming an upper magnetic layer by applying a magnetic layer coating material containing at least a ferromagnetic powder having an average long axial length of 0.1 &mgr;m or less and a binder resin onto the lower non-magnetic layer, and then drying the coating material;

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